84 



Cellular Structure and Activity 



ENZYME CONCENTRATION IN 

 EMBRYONIC LIFE 



The most straightforward way in which 

 embryonic metabolism is altered is through 

 changes in relative concentrations of differ- 

 ent enzymes. Among the few studies clearly 

 bearing on this problem are a sufficient num- 

 ber on one form, the chick embryo,* to 

 enable us to state with assurance that em- 

 bryonic tissue at every stage is enzymatically 

 individual, rather than being merely a sim- 

 plified or diluted version of any adult type 

 (cf. Moog, '52). During the first few days 

 of development, the tissue of the embryonic 

 chick is relatively poor in phosphatases 

 (Moog and Steinbach, '46; Moog, '46); rela- 

 tively rich in various peptidases (Levy and 

 Palmer, '43); and very rich in cytochrome 

 oxidase and succinoxidase [this fact is cal- 

 culated in part from the data of Albaum et 

 al. ('46) who, however, reported their find- 

 ings only in terms of total quantity per 

 embryo]. Interestingly enough, the period 

 of intensive histogenetic activity beginning 

 at the end of the first week of incubation 

 is preceded by a slackening in rate of pro- 

 duction of all enzymes so far studied. 



Finding a differential increase in enzyme 

 activity per unit of embryo by the homog- 

 enate method may, of course, mean only that 

 a given tissue rich in the enzyme in ques- 

 tion is increasing faster than other tissues. 

 Undoubtedly this is the explanation for nu- 

 merous cases of enzyme increase per unit of 

 whole organism. But it has by now been 

 amply demonstrated that differential changes 

 in enzyme concentrations also occur in iso- 

 lated tissues; in the brain of the honey bee, 

 in fact, cholinesterase rises while cell num- 

 ber declines (cf. Rockstein, '50). Studies like 

 those of Sawyer ('43a, '43b) on cholines- 

 terase in muscle and nerve, Hermann and 

 Nicholas ('48) on apyrase in muscle, and 



* This section will be largely, but not wholly, 

 confined to the chick embryo. By reason of easy 

 availability and relative purity of cytoplasm, the 

 chick embryo seems better adapted to the type of 

 research we are dealing with here than any other 

 form. The enzymology of amphibian embryos has 

 been much studied, but in these the difficulty of 

 separating inert yolk from active cytoplasm, or of 

 determining how fast one is converted into the 

 other, has meant that there has been no satisfactory 

 unit basis to which enzyme activity could be re- 

 ferred. Gregg and Lovtrup ('50) have, however, 

 recently proposed a method for determining non- 

 yolk nitrogen in salamander eggs. At the present 

 time, no form has been explored fully enough to 

 serve as the sole basis of even a brief resume of 

 embryonic metabolism. 



Moog ('50, '51) on phosphatase in intestine, 

 seem to be establishing as a fact the repeated 

 finding that in advanced stages of develop- 

 ment an enzyme accumulates only in cor- 

 relation with the function it subserves — ■ 

 either in parallel with the function, or in 

 slight forward reference to it. 



If this view is correct, it will serve as a 

 valuable guide in examining the poorly ex- 

 plored enzymology of the early stages of 

 development. It is in these early stages 

 (roughly the first six days in the life of the 

 chick, for example) ihat we can with ap- 

 proximate correctness speak of "embryonic 

 tissues" as a type, in contrast to the partly 

 or fully differentiated tissues of later stages. 

 Finding that enzymes do not behave inde- 

 pendently of their correlated function may 

 justify our concluding that some, at least, 

 of the enzymes of very young embryonic 

 tissue are actively related to the proper func- 

 tion of such tissue, i.e., development. 



With embr>^onic material just as with 

 adult, values for enzyme activity obtained for 

 homogenates or extracts cannot be uncriti- 

 cally accepted as reflecting the state of affairs 

 in vivo. Quite possibly, however, the enzyme 

 activities obtained with embryonic homog- 

 enates are generally closer to the activities 

 in intact tissue than is the case with adult 

 material. As pointed out before, the embry- 

 onic cell might be expected not to store a 

 large reserve of enzymes for emergency use, 

 since the orderly nature of embryonic life 

 is itself a guarantee against the stresses that 

 the adult must face. 



ENZYME ACTIVITY IN EMBRYONIC 

 LIFE 



The Influence of Substrate and Cofactor 

 Concentration. That the activity engaged in 

 by a given number of enzyme molecules may 

 be limited by availability of substrate or of 

 cofactors has already been pointed out. It 

 might be supposed, then, that the rate at 

 which raw materials are supplied, by the 

 yolk sac or placenta or other commissary 

 agency, would control the rate at which bio- 

 chemical reactions go forward within the 

 embryonic body. At the present time, how- 

 ever, no good evidence bears on this prob- 

 lem. One might call to mind the old obser- 

 vation reported by Needham ('31), that the 

 tendency of the chick embryo to use its 

 energy sources in succession — carbohydrate 

 first, then protein, then fat — is not altered 

 by the injection of large amounts of glu- 

 cose at the period when protein is in prin- 



